Wireless local area networks (LANs) have been developed as an enhanced replacement for wired LANs. In a wireless LAN for data-communication a plurality of (mobile) network stations (e.g., personal computers, telecommunication devices, etc.) are present that are capable of wireless communication. As compared to wired LANs, data-communication in a wireless LAN can be more versatile, due to the flexibility of the arrangement of network stations in the area covered by the LAN, and due to the absence of cabling connections.
Wireless LANs are generally implemented according to the standard as defined by the ISO/IEC 8802–11 international standard (IEEE 802.11). IEEE 802.11 describes a standard for wireless LAN systems that will operate in the 2.4–2.5 GHz ISM (industrial, scientific and medical) band. This ISM band is available worldwide and allows unlicensed operation for spread spectrum systems. For both the US and Europe, the 2,400–2,483.5 MHz band has been allocated, while for some other countries, such as Japan, another part of the 2.4–2.5 GHz ISM band has been assigned. The IEEE 802.11 standard focuses on the MAC (medium access control) and PHY (physical layer) protocols for AP based networks and ad-hoc networks.
In AP based wireless networks, the stations within a group or cell can communicate only directly to the AP. This AP forwards messages to the destination station within the same cell or through the wired distribution system to another AP, from which such messages arrive finally at the destination station. In ad-hoc networks, the stations operate on a peer-to-peer level and there is no AP or (wired) distribution system.
The 802.11 standard supports three PHY protocols: DSSS (direct sequence spread spectrum), FHSS (frequency hopping spread spectrum), and infrared with PPM (pulse position modulation). All these three PHYs provide bit rates of 1 and 2 Mbit/s. Furthermore, IEEE 802.11 includes extensions 11a and 11b which allow for additional higher bit rates: Extension 11b provides bit rates 5.5 and 11 Mbit/s as well as the basic DSSS bit rates of 1 and 2 Mbit/s within the same 2.4–2.5 GHz ISM band. Extension 11a provides a high bit rate OFDM (Orthogonal Frequency Division Multiplexing modulation) PHY standard providing bit rates in the range of 6 to 54 Mbit/s in the 5 GHz band.
The IEEE 802.11 basic MAC protocol allows interoperability between compatible PHYs through the use of the CSMA/CA (carrier sense multiple access with collision avoidance) protocol and a random back-off time following a busy medium condition. The IEEE 802.11 CSMA/CA protocol is designed to reduce the collision probability between multiple stations accessing the medium at the same time. Therefore, a defer and random back-off arrangement is used to resolve medium contention conflicts. The defer decision is based on a configuration entity called the defer threshold (R_defer). When a carrier signal level is observed above the R_defer level, a network station holds up a pending transmission request. If the observed level is below the R_defer, a network transmission is allowed to start communicating with its associated access point.
In addition, the IEEE 802.11 MAC protocol defines special functional behaviour for fragmentation of packets, medium reservation via RTS/CTS (request-to-send/clear-to-send) polling interaction and point co-ordination (for time-bounded services).
Moreover, the IEEE 802.11 MAC protocol defines Beacon frames sent at regular intervals by the AP to allow stations to monitor the presence of the AP.
The IEEE 802.11 standard defines two types of MAC mechanisms: PCF (point co-ordination function) which provides contention free frame transfer whereas DCF (distributed co-ordination function) provides contention based frame transfer. Both these MAC mechanisms can operate together. This is done by dividing the time between two beacons into a contention free part (PCF) and a contention part (DCF). The CFP (Contention Free Period) repetition interval is a fixed length which includes both the contention free period as well as contention period. See also FIG. 59 in the IEEE 802.11 standard.
The IEEE 802.11 MAC protocol also gives a set of management frames including Probe Request frames, which are sent by a station and are followed by Probe Response frames sent by an available AP. This protocol allows a station to actively scan for APs operating on other frequency channels and for the APs to show to the stations what parameter settings the APs are using. In 802.11 AP-based wireless LANs the network stations normally associate to an AP that is the best received and the nearest and has a corresponding network name.
Every DSSS AP operates on one channel. The number of channels depends on the regulatory domain in which the wireless LAN is used (e.g. 11 channels in the U.S. in the 2.4 GHz band). This number can be found in ISO/IEC 8802-11, ANSI/IEEE Std 802.11 Edition 1999-00-00. Overlapping cells using different channels can operate simultaneously without interference if the channel distance is at least 3. Non-overlapping cells can always use the same channels simultaneously without interference. Channel assignment can be dynamic or fixed. Dynamic channel assignment is preferable, as the environment itself is dynamic as well.
In [Kamerman, December 1999] dynamic assignment of channels is called dynamic frequency selection (DFS). The aim of the DFS algorithm is to dynamically assign channels in a wireless LAN in such a way that the best performance is achieved. Performance can be expressed in terms of throughput, delay and fairness. An AP with dynamic frequency selection is able to switch its channel in order to obtain a better operating channel. It will usually choose a channel with less interference and channel sharing than that on the current channel. An AP will scan on all channels to determine which channel frequencies are in use and what receive levels and load factors occur in neighbour cells. During a scan of a channel the AP sends a Probe Request frame to evoke a Probe Response from all APs tuned to the same channel and within radio range. The Probe Response packet carries information on load factor from each AP on the channel in question.
By scanning over all channels, an AP assembles a table with an entry for each channel. Each entry contains receive level, the load factor as reported in the Probe Response packet and the measured noise level. The receive level stored in the table is the level at which the Probe Response packet is received from another AP active operating on the channel in question. The said table is used in a DFS algorithm as described in [Kamerman, December 1999].
The strategy of the DFS algorithm described in [Kamerman, December 1999] expects responding APs to send load information in the Probe Responses, which is not standard (IEEE 802.11) compliant. So it is very likely that this load information will never be obtained from APs made by other manufacturers. Therefore it does not solve the problem of unlicensed spectrum. Secondly, waiting for the Probe Requests can take up to 50 ms if the other AP is very busy. This situation is highly undesirable especially if the AP sending the Probe Request is highly loaded. The load at the AP sending the Probe Request is not taken into account while periodic scanning. Thirdly, the strategy as described in [Kamerman, 1999] lacks a strategy about when to change channels. Changing channels is done periodically but this might not be necessary at all. Finally, in the DFS algorithm mentioned above, a fixed scan interval of 1 hour is used. This is a very long time and a lot of changes in circumstances can take place. A microwave oven could come on and go off within that time, causing a decrease in throughput of an AP. On the other hand, if the scanning interval is reduced to a very small value, the AP could be scanning for most of the time causing again a decrease in throughput. The problem lies in the fixed scan interval and the fact that all channels are scanned one after the other.